Trunked radio systems are one solution for permitting two-way radio communications, such as voice and data communications, not only between individual users equipped with subscriber radio units, but also among all active members of particular talkgroups. Conventional two-way radio systems permit group communications among radio users who tune in to a particular communications channel, such as a particular frequency, that has been assigned in advance. Trunked radio systems, on the other hand, do not require members of a talkgroup to select any particular communications channel in advance. Instead, communications channels are assigned only as they are needed. The assignment of communications channels is performed by a site controller. Users of subscriber units remain in wireless communication with a site controller over a duplex control channel, which is comprised of an inbound channel for communications to the site controller and an outbound channel for communications from the site controller to the subscriber units.
When a communications channel is needed for talkgroup or individual communications, a request is made to the site controller over the inbound control channel. Over the outbound control channel, the site controller alerts the relevant subscriber units to the communications channel to be used.
Subscriber units can be turned on or off, or they can enter and leave an area served by a particular site controller, and site controllers themselves can be in and out of service at various times. To keep track of which subscriber units are being served by which site controller, subscriber units engage in a registration process in which subscriber units send registration requests to a site controller and identify the talkgroup or groups with which they are affiliated. In turn, the site controller registers the presence of the subscriber units and confirms the registration by sending registration grants to the respective subscriber units. A site controller may also be affiliated with one or more relays that allow subscriber units to communicate with one another despite being served by different site controllers. Such communications may take place through the intermediary of a backend network.
Trunked radio systems have several advantages over conventional radio systems, such as a more efficient allocation of the radio spectrum and greater flexibility in the assignment of subscriber units to multiple different talkgroups. However, the use of a trunked radio system poses other challenges. For example, it is possible for the control channel, which can be shared by numerous subscriber units, to be overloaded by radio traffic. Such a situation can arise when several subscriber units are attempting to send registration requests at the same time. When two subscriber units attempt to send messages, such as registration requests, on the same channel at the same time, it is often the case that neither message is intelligible. The two subscriber units whose requests have collided then need to make a new attempt to send their messages. It is known in the art for the subscriber units whose messages have collided to employ a random back-off routine and to make a new attempt to send the message after a random delay period has passed. Since both subscriber units are not likely to select the same random delay period, the messages are not likely to collide in the second attempt.
The use of a random back-off routine works well during ordinary radio traffic conditions. However, on occasion, a situation can arise in which the amount of subscriber unit registration traffic is too high to be satisfactorily handled by only a random back-off routine. One such situation is a mass registration event, where some occurrence leads to a large number of registration requests to be sent at substantially the same time. For example, a situation may arise in which one site controller fails, leading many of the subscriber units that were formerly registered with that site controller to inundate neighboring site controllers with new registration requests.
Accordingly, there is a need for an improved method and system for managing a mass registration scenario in a two-way radio system.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.